Method for making carboxylic acids

ABSTRACT

The present invention relates to a process for producing carboxylic acids. It relates more particularly to a process for producing carboxylic acids by oxidation of a hydrocarbon with oxygen or a gas containing oxygen, and even more particularly to the oxidation of cyclohexane to adipic acid. The invention relates to a process for producing carboxylic acids by oxidation with oxygen or a gas containing oxygen of a cycloaliphatic hydrocarbon in the presence of an oxidation catalyst and of a monocarboxylic oxidation solvent that is lipophilic in nature, comprising a step of extraction of the dicarboxylic acids formed in the oxidation step, consisting in carrying out, in liquid phase, an extraction of the diacids using water.

The present invention relates to a process for producing carboxylicacids.

It relates more particularly to a process for producing carboxylic acidsby oxidation of a hydrocarbon with oxygen or a gas containing oxygen,and even more particularly to the oxidation of cyclohexane to adipicacid.

Adipic acid is an important chemical compound used in many fields. Thus,adipic acid can be used as an additive in many products, both in thearea of foods and in concrete. However, one of the most important usesis its application as a monomer in the production of polymers, includingpolyurethanes and polyamides.

Several processes for producing adipic acid have been proposed. One ofthe most important, used industrially on a large scale, consists inoxidizing, in one or two step(s), cyclohexane to a mixture ofcyclohexanol/cyclohexanone with a gas containing oxygen or with oxygen.After extraction and purification of the cyclohexanol/cyclohexanonemixture, these compounds are oxidized in particular to adipic acid withnitric acid. However, this process has a major drawback associated withthe formation of nitrous vapour.

Many studies have been carried out in order to develop a process foroxidizing hydrocarbons with oxygen or a gas containing oxygen, thatmakes it possible to directly obtain carboxylic acids, mainly adipicacid.

These processes are described in particular in patents FR 2,761,984, FR2,791,667, FR 2,765,930 and U.S. Pat. No. 5,294,739.

Generally, the reaction is carried out in a solvent medium, the solventbeing a monocarboxylic acid such as acetic acid. However, such a processhas not, for the moment, been the subject of any substantial industrialdevelopment since the separation of adipic acid and of acetic acidrequires considerable process steps in order to obtain, firstly, ahighly pure adipic acid compatible with the demands required for theproduction of polyamide, in particular for textile applications, and,secondly, recovery and recycling of the acetic acid that are as completeas possible so as not to economically penalize the process.

In order to attempt to remedy these drawbacks, novel solvents have beensought and proposed for carrying out the oxidation of cyclohexane withoxygen. Thus, monocarboxylic acid solvents that are lipophilic in naturehave been proposed, in particular in patent FR 2806079. The acids havethe advantage of a low affinity with adipic acid and of not beingsoluble in solvents for adipic acid, such as water. Consequently, therecovery of the adipic acid in the aqueous phase present at the end ofthe oxidation step can be carried out more readily with limitedentrainment of the mono-carboxylic solvent.

However, these solvents can have high melting points, in particularabove ambient temperature, which means a more complex extraction processor that the extraction or separation processes must be carried out athigh temperatures.

One of the aims of the present invention is to propose an effective andeasy process for producing carboxylic acids by oxidation of hydrocarbonsusing oxygen or a gas containing oxygen, in the presence of a solventthat is lipophilic in nature, which process uses a step consisting ofeffective extraction of the acids formed and complete recycling of theoxidation solvent.

To this effect, the invention proposes a process for producingdicarboxylic acids by oxidation with oxygen or a gas containing oxygenof a cycloaliphatic hydrocarbon in the presence of an oxidation catalystand of an oxidation solvent that is lipophilic in nature, characterizedin that it comprises a step of extraction of the dicarboxylic acidsformed in the oxidation step, consisting in carrying out, in liquidphase, an extraction of the diacids using an extraction solvent in whichat least the oxidation solvent and the cycloaliphatic hydrocarbon areinsoluble.

As a solvent that is lipophilic in nature, monocarboxylic compounds arepreferred.

For the purpose of the patent, the products are considered to beinsoluble in the extraction solvent if their solubility in said solvent,measured at 90° C. and under atmospheric pressure, is less than or equalto 10% by weight relative to the solvent.

According to one characteristic of the invention, the extraction of thediacids formed is carried out in a countercurrent-flow liquid/liquidextraction column. The extraction solvent is advantageously chosen fromthe group comprising polar solvents, water and alcohols such asmethanol. The preferred solvent is water or a solution containing mainlywater.

As liquid-liquid extraction columns that are suitable for the invention,the various principles and devices commonly used in industrial processescan be used. Thus, columns with mechanical agitation using a disc orrotor, columns using pulse technology, static columns with perforatedtrays or static packed columns are suitable. Preferably, columns withmechanical agitation are preferred. Of course, this extraction can becarried out in a single extraction column or in several extractioncolumns mounted in series and/or in parallel without neverthelessdeparting from the context of the invention. It is also possible to useone or more extraction columns in combination with washing-settlingdevices.

According to a preferred novel characteristic of the invention, thereaction medium derived from the oxidation step is fed into theextraction step under given temperature and pressure conditions so as tomaintain the cycloaliphatic hydrocarbon in the liquid state.

Advantageously, the extraction of the diacids is carried out under giventemperature and pressure conditions so as to maintain the cycloaliphatichydrocarbon in the liquid state.

The maintaining of the hydrocarbon in the liquid state during theextraction phase makes it possible to maintain the oxidation solvent inthe solubilized state, or to maintain a homogeneous solution between thehydrocarbon and the oxidation solvent. Thus, it is possible to carry outthe extraction of the diacids under temperature conditions that are lesssevere, in particular within a temperature range below the solidifyingtemperature or crystallization temperature of the oxidation solvent, orat a temperature that prevents any precipitation of the oxidationsolvent.

In a particular embodiment of the invention, a second extraction solventis fed into the extraction column in a direction countercurrent to thefirst extraction solvent. This second solvent is a solvent for theoxidation solvent such as monocarboxylic acids that are lipophilic innature and is not miscible with the first extraction solvent. Thisdouble extraction allows a virtually complete recovery of the oxidationsolvent and recycling thereof.

This second solvent is chosen from apolar solvents having a solubilityin the first extraction solvent of less than or equal to 5% by weightrelative to the first extraction solvent, this solubility being measuredat a temperature of 20° C. under atmospheric pressure. Advantageous,this second solvent is chosen from acyclic or cyclic, saturatedhydrocarbons, and aromatic hydrocarbons. Advantageously, this secondsolvent is the hydrocarbon to be oxidized, in particular cyclohexane.Advantageously, the supply of this second solvent represents at leastpartially the supply of hydrocarbon to be oxidized in the oxidationprocess of the invention when it is a continuous process.

In a preferred embodiment of the invention, the reaction medium issupplied at an intermediate position on the column, and the firstextraction solvent and the second extraction solvent are supplied,respectively, at each end of the column.

The reaction medium is generally obtained from the oxidation, withoxygen or a gas containing oxygen, of an arylaliphatic hydrocarbon, moreparticularly of a cycloaliphatic arylaliphatic hydrocarbon such ascyclohexane or cyclododecane. The oxidation reaction is generallycarried out in the presence of a solvent. The solvent may be very variedin nature insofar as it is not substantially oxidizable under thereaction conditions.

According to a preferred characteristic of the invention, the solvent ischosen from carboxylic acids that are lipophilic in nature.

The expression “lipophilic acid compound that is suitable for theinvention” is intended to mean aromatic, aliphatic, arylaliphatic oralkylaromatic compounds comprising at least 6 carbon atoms, that maycomprise several acid functions and that have low water-solubility, i.e.a solubility of less than 10% by weight at ambient temperature (10°C.-30° C.).

As lipophilic organic compounds, mention may be made, for example, ofhexanoic acid, heptanoic acid, octanoic acid, 2-ethylhexanoic acid,nonanoic acid, decanoic acid, undecanoic acid, dodecanoic acid, stearicacid (octadecanoic acid) and their permethylated derivatives (completesubstitution of the hydrogens of the methylene groups with the methylgroup), 2-octadecylsuccinic acid, 3,5-ditert-butyl-benzoic acid,4-tert-butylbenzoic acid, 4-octylbenzoic acid, tert-butyl hydrogenorthophthalate, naphthenic or anthracenic acids substituted with alkylgroups, preferably of tert-butyl type, substituted derivatives ofphthalic acids, and fatty diacids such as dimer fatty acid. Mention mayalso be made of the acids belonging to the above families and bearingvarious electron-donating substituents (groups with a hetero atom of theO or N type) or electron-withdrawing substituents (halogens,sulphonimides, nitro groups, sulphonato groups, or the like).

In general, the lipophilic acid solvent is chosen so as toadvantageously obtain a homogeneous phase under the temperature andpressure conditions at which the oxidation reaction is carried out. Forthis, it is advantageous for the solubility of the solvent in thehydrocarbon or the reaction medium to be at least greater than 2% byweight, and for at least one homogeneous liquid phase comprising atleast some of the hydrocarbons to be oxidized and some of the solvent tobe formed.

Advantageously, the solvent is chosen from those with lowwater-solubility, i.e. that have a water-solubility of less than 10% byweight at ambient temperature (10-30° C.).

However, it is possible, without departing from the context of theinvention, to use a solvent having a water-solubility that is greaterthan that indicated above, if the partition coefficient for thiscompound between the organic phase(s) of the reaction medium consistingessentially of the hydrocarbon to be oxidized, the oxidationintermediates and the nonorganic phase comprising the water formedduring the oxidation reaction makes it possible to obtain aconcentration of the solvent in said aqueous phase of less than 10% byweight.

The oxidation is in general carried out in the presence of a catalyst.This catalyst advantageously comprises a metal element chosen from thegroup comprising Cu, Ag, Au, Mg, Ca, Sr, Ba, Zn, Cd, Hg, Al, Sc, In, Tl,Y, Ga, Ti, Zr, Hf, Ge, Sn, Pb, V, Nb, Ta, Cr, Mo, W, Mn, Tc, Re, Fe, Ru,Os, Co, Rh, Ir, Ni, Pd, Pt, lanthanides such as Ce, and combinationsthereof.

These catalytic elements are used either in the form of compounds thatare advantageously at least partially soluble in the liquid oxidationmedium under the conditions under which the oxidation reaction iscarried out, or are carried by, absorbed onto or attached to an inertsupport such as silica or alumina, for example.

The catalyst is preferably, in particular under the conditions underwhich the oxidation reaction is carried out:

-   -   either soluble in the hydrocarbon to be oxidized,    -   or soluble in the lipophilic acid compound,    -   or soluble in the hydrocarbon/lipophilic acid compound mixture        forming a homogeneous liquid phase under the conditions under        which the reaction is carried out.

According to a preferred embodiment of the invention, the catalyst usedis soluble in one of these media at ambient temperature or at thetemperature for recycling of these media in a further oxidation.

The term “soluble” is intended to mean that the catalyst is at leastpartially soluble in the medium under consideration.

In the case of a heterogeneous catalyst, the catalytically active metalelements are supported or incorporated in or into a microporous ormesoporous mineral matrix, or in or into a polymeric matrix, or are inthe form or organometallic complexes grafted onto an organic or mineralcarrier. The term “incorporated” is intended to mean that the metal isan element of the carrier or that the process is carried out withcomplexes that are sterically trapped in porous structures under theconditions of the oxidation.

In a preferred embodiment of the invention, the homogeneous orheterogeneous catalyst consists of salts or complexes of metals ofgroups Ivb (the group of Ti), Vb (the group of V), VIb (the group ofCr), VIIb (the group of Mn), VIII (the group of Fe or Co or Ni) and Ib(the group of Cu) and cerium, alone or as a mixture. The preferredelements are, in particular, Mn and/or Co which can be used incombination with one or more elements chosen from the group comprisingCr, Zr, Hf, Ce and Fe. The concentrations of metal in the liquidoxidation medium range between 0.00001 and 5% (wt %), preferably between0.001% and 2%.

Moreover, the concentration of solvent in the reaction medium isadvantageously determined so as to have a molar ratio of the number ofmolecules of solvent and the catalytic element metal number of between0.5 and 100 000, preferably between 1 and 5000.

The concentration of solvent in the liquid oxidation medium can varywithin broad limits. Thus, it can be between 1 and 99% by weightrelative to the total weight of liquid medium, more advantageously itcan be between 2 and 50% by weight of the liquid medium.

It is also possible, without nevertheless departing from the context ofthe invention, to use the solvent in combination with another compoundthat may in particular have the effect of improving the productivityand/or the selectivity of the reaction of oxidation to adipic acid, andin particular the solubilization of the oxygen.

As examples of such compounds, mention may in particular be made ofnitrites, hydroxyimide compounds, halogenated compounds, and moreadvantageously fluorinated compounds. As compounds that are moreparticularly suitable, mention may be made of nitrites such asacetonitrile or benzonitrile, imides belonging to the family describedin European Patent EP 0824962, and more particularlyN-hydroxysuccinimide (NHS) or N-hydroxyphthalimide (NHPI), halogenatedderivatives such as dichloromethane, and fluorinated compounds such as:

-   -   cyclic or acyclic, fluorinated or perfluorinated, aliphatic        hydrocarbons,    -   aromatic fluorinated hydrocarbons such as perfluorotoluene,        perfluoromethylcyclohexane, perfluoroheptane, perfluorooctane,        perfluorononane, perfluorodecaline, perfluoromethyldecaline,        α,α,α-trifluorotoluene or 1,3-bis-(trifluoromethyl)benzene,    -   perfluorinated or fluorinated esters such as alkyl        perfluorooctanoates or alkyl perfluorononanoates,    -   fluorinated or perfluorinated ketones such as perfluoroacetone,    -   fluorinated or perfluorinated alcohols such as perfluorohexanol,        perfluorooctanol, perfluorononanol, perfluorodecanol,        perfluoro-tert-butanol, perfluoroisopropanol or        1,1,1,3,3,3-hexafluoro-2-propanol,    -   fluorinated or perfluorinated nitriles such as        perfluoroacetonitrile,    -   fluorinated or perfluorinated acids such as        trifluoromethylbenzoic acids, pentafluorobenzoic acid,        perfluorohexanoic acid, perfluoroheptanoic acid,        perfluorooctanoic acid, perfluorononanoic acid or        perfluoroadipic acid,    -   fluorinated or perfluorinated halides such as        perfluoroiodooctane, or perfluorobromooctane,    -   fluorinated or perfluorinated amines such as        perfluorotripropylamine, perfluorotributylamine or        perfluorotripentylamine.

The invention applies more particularly to the oxidation ofcycloaliphatic compounds, such as cyclohexane or cyclododecane, to thecorresponding linear diacids, adipic acid or dodecanoic acid.

According to a preferred embodiment of the invention, it relates to thedirect oxidation of cyclohexane to adipic acid, with a gas containingoxygen, in a liquid medium and in the presence of a manganese catalyst,in particular a manganese- and cobalt-based catalyst.

The oxidation reaction is carried out at a temperature of between 50° C.and 200° C., preferably of between 70° C. and 180° C. It can be carriedout under atmospheric pressure. However, it is generally carried outunder a pressure so as to maintain the components of the reaction mediumin the liquid form. The pressure can be between 10 kPa (0.1 bar) and 20000 kPa (200 bar), preferably between 100 kPa (1 bar) and 10 000 kPa(100 bar).

The oxygen used may be in pure form or as a mixture with an inert gassuch as nitrogen or helium. Air more or less enriched with oxygen mayalso be used. The amount of oxygen supplied to the medium isadvantageously between 1 and 1000 mol per mole of compounds to beoxidized.

The oxidation process can be carried out continuously or according to abatch process. Advantageously, the liquid reaction medium that has leftthe reactor is treated according to known processes for, firstly,separating and recovering the diacid produced and, secondly, recyclingthe non-oxidized or partially oxidized organic compounds such ascyclohexane, cyclohexanol and/or -cyclohexanone, the catalyst and theacid compound.

It is advantageous to also use a compound that initiates the oxidationreaction, for instance a ketone, an alcohol, an aldehyde or ahydroperoxide. Cyclohexanone, cyclohexanol and cyclohexyl hydroperoxide,which are reaction intermediates in the case of the oxidation ofcyclohexane, are all particularly indicated. In general, the initiatorrepresents from 0.01% to 20% by weight of the weight of the reactionmixture used, without these proportions having a critical value. Theinitiator is especially useful during the initiation of the oxidation.It can be introduced from the beginning of the reaction.

The oxidation can also be carried out in the presence of waterintroduced from the initial stage of the process.

In these various embodiments, the carboxylic acid recovered after theliquid/liquid extraction step can be purified according to the usualtechniques described in many documents, for example by crystallizationand recrystallization from various solvents such as water, acetic acidor other organic solvents. Purification processes are in particulardescribed in French Patents No. 2,749,299 and 2,749,300.

Similarly, if the catalyst is not entirely recycled with the organicphase, and is partly or completely extracted with the aqueous phase, itwill be advantageously extracted from the aqueous phase by varioustechniques, such as liquid/liquid extraction, electrodialysis, ortreatment on ion exchange resin, for example.

Moreover, the organic phase recovered from the reaction medium can besubjected to distillation operations in order to recycle the nonoxidizedhydrocarbon, the various oxidation compounds such as the alcohols, theketones, and the oxidation solvent. In addition, the organic phase canbe subjected to a treatment in order to eliminate the esters, inparticular before recycling of the solvent.

Other advantages and details of the invention will become apparent inview of the examples given below, only by way of indication.

EXAMPLE 1

Oxidation Step

522 g of cyclohexane, 55 g of tert-butylbenzoic acid and 6 g ofcyclohexanone (initiator) are placed in a 1.5 l reactor. Manganese andcobalt are added in respective amounts of 50 and 20 ppm by mass.

The mixture is stirred at 130° C., 20 bar, for 150 min under acontinuous stream of gas containing nitrogen and oxygen. After 35 l ofoxygen have been consumed, a mixture of cyclohexane, oftert-butylbenzoic acid, of cyclohexanone, of cyclohexanol, of manganeseand of cobalt is added continuously. A level sensor connected to areactor emptying system makes it possible to keep the reactor levelconstant.

Extraction Step

After 3 hours of stabilized regime, a portion of 200 g of oxidationreaction mixture is isolated. A mass of 200 g of water is added to thisfraction in a mixer maintained at 70° C. After agitation and thenseparation by settling out, two phases are recovered: a lower phase,referred to as aqueous phase, which contains essentially the diacidsproduced and the catalytic metals, and an upper phase, referred to asorganic phase, which contains essentially cyclohexane, thetert-butylbenzoic acid, cyclohexanone, cyclohexanol and otherby-products of the reaction.

Analysis of the two phases shows that 85% by weight of the adipic acidformed and present in the reaction mixture portion is recovered andextracted in the aqueous phase, and 99.3% by weight of the t-BBA isrecovered in the organic phase.

EXAMPLE 2

The oxidation step is identical to that described in Example 1. However,the oxidation reaction medium withdrawn continuously from the reactor isfed into an agitated stage extraction column having a theoretical stagenumber of 10. The extraction column operates at a temperature of 100° C.under a pressure of 5 bar.

The column is fed at the level of an intermediate stage (fifth stage)with the oxidation reaction medium, with a flow rate corresponding to afeed of 1.11 kg/h of t-BBA and 1.09 kg/h of adipic acid at a temperatureof 100° C. under a pressure of 5 bar.

The column is also fed, at the head of the column, with water with aflow rate of 1.62 kg/h and, at the foot of the column, with a flow ofcyclohexane equal to 1.04 kg/h.

The organic phase recovered at the head of the column comprises 1.11kg/h of t-BBA, 1.04 kg/h of cyclohexane and 0.007 kg/h of adipic acid.

The aqueous phase withdrawn at the foot of the column comprises 0.001kg/h of t-BBA, 1.62 kg/h of water and 1.083 kg/h of adipic acid.

These trials clearly demonstrate that the process of the invention makesit possible to extract virtually all the adipic acid formed withoutentraining oxidation solvent. In fact, said solvent is almost entirelyin the organic phase and may be recycled after, advantageously, apurification.

In addition, some of the adipic acid may be entrained in the organicphase in the form of esters. These may be treated before the recyclingof the oxidation solvent.

1-19. (canceled)
 20. A process for producing dicarboxylic acids byoxidation with oxygen or a gas containing oxygen of a cycloaliphatichydrocarbon in the presence of an oxidation catalyst and of an oxidationsolvent that is lipophilic in nature, comprising the step of extractingthe dicarboxylic acids formed in the oxidation step by carrying out, inliquid phase, an extraction of the diacids using a first extractionsolvent in which at least the oxidation solvent and the cycloaliphatichydrocarbon are insoluble.
 21. A process according to claim 20, whereinthe oxidation solvent that is lipophilic in nature is a monocarboxylicacid.
 22. A process according to claim 20, wherein the extraction of thediacids is carried out in a countercurrent-flow liquid/liquid extractioncolumn.
 23. A process according to claim 20, wherein the reaction mediumderived from the oxidation step is fed into the extraction step undergiven temperature and pressure conditions so as to maintain thecycloaliphatic hydrocarbon in the liquid state.
 24. A process accordingto claim 20, wherein the extraction of the diacids is carried out undergiven temperature and pressure conditions so as to maintain thecycloaliphatic hydrocarbon in the liquid state.
 25. A process accordingto claim 20, wherein the first extraction solvent is a polar solventwhich is water or an alcohol.
 26. A process according to claim 25,wherein the first extraction solvent is water.
 27. A process accordingto claim 20, further comprising the addition of a second extractionsolvent to the extraction step, said second extraction solvent beingnon-miscible with the first extraction solvent, and not solubilizing thediacids formed.
 28. A process according to claim 27, wherein the firstand the second extraction solvents are fed into the countercurrentextraction column.
 29. A process according to claim 27, wherein thesecond extraction solvent is an acyclic hydrocarbon, a cyclichydrocarbon, a saturated hydrocarbon, or an aromatic hydrocarbon.
 30. Aprocess according to claims 27, wherein the second extraction solvent isthe cycloaliphatic hydrocarbon to be oxidized.
 31. A process accordingto claim 22, wherein the oxidation medium is fed into the extractioncolumn at an intermediate position between the two ends of the column.32. A process according to claim 20, wherein the hydrocarbon is acycloalkane.
 33. A process according to claim 20, wherein thecycloalkane is cyclohexane or cyclododecane.
 34. A process according toclaim 20, wherein the solvent is a monocarboxylic acid that islipophilic in nature, having from 7 to 20 carbon atoms.
 35. A processaccording to claim 20, wherein the lipophilic acid is hexanoic acid,heptanoic acid, octanoic acid, 2-ethylhexanoic acid, nonanoic acid,decanoic acid, undecanoic acid, dodecanoic acid, stearic acid(octadecanoic acid), 2-octadecylsuccinic acid, 1,5-ditert-butylbenzoicacid, 4-tert-butylbenzoic acid, 4-octylbenzoic acid, tert-butyl hydrogenorthophthalate, a naphthenic acid substituted with alkyl group, aanthracenic acid substituted with alkyl groups, a substitutedderivatives of a phthalic acid, or a fatty diacid.
 36. A processaccording to claim 20, wherein the catalyst is a transition metal.
 37. Aprocess according to claim 36, wherein the catalyst is based onmanganese in combination with a co-catalyst which is cobalt, chromium,zirconium, hafnium or iron alone or in combination.
 38. A processaccording to claim 20, wherein the dicarboxylic acids produced areadipic acid, succinic acid, glutaric acid, dodecanedioic acid or amixture thereof.